Sucrose may be converted to glucose-fructose either enzymatically with invertase or through acid hydrolysis. Industrial processes for producing glucose-fructose solutions from sucrose generally utilize an acid hydrolysis process which is expensive. Enzymatic processes generally require preparation of a biocatalyst comprised of the invertase enzyme or an invertase-producing microorganism immobilized on a support to prevent the enzyme or microorganism from being washed from the reactor. Presently known processes for producing such immobilized biocatalysts are not suitable for industrial use.
One process, described by Leuba, et al., in U.S. Pat. No. 4,918,016, discloses invertase immobilized on a chitosan-coated silica gel support and retaining significant enzymatic activity for converting sucrose to glucose-fructose. Leuba, et al. do not disclose use of their immobilized invertase in a continuous-flow reactor that might have industrial use. Another process described by Lazcano, et al., in U.S. Pat. No. 5,270,177, utilizes a recombinant yeast strain with the microorganism immobilized in calcium alginate. The process for producing the immobilized microorganism requires a specially-built needle device for dripping a sodium alginate solution containing the microorganism over a calcium chloride solution. Further, the alginate is relatively expensive for industrial use. Finally, the temperature range of the biocatalyst and method disclosed in Lazcano, et al. is limited to 40-50.degree. C., further limiting industrial applications.
Numerous other methods for preparing and using immobilized microorganisms or enzymes have been described such as the methods and uses described by Colilla, et al., in U.S. Pat. No. 4,111,750; by Sipos in U.S. Pat. No. 3,708,397; by Lommi, et al., in U.S. Pat. No. 5,079,011, by Harder, et al., in U.S. Pat. No. 5,405,764, and by Harder, et al., in U.S. Pat. No. 5,314,814.
The methods known for immobilizing invertase or invertase-producing microorganisms for use as biocatalysts in the production of fructose are impractical for use on an industrial scale because such methods generally require expensive equipment or materials or involve complex processes. Moreover, in some existing methods, the support onto which the enzyme or microorganism is immobilized is easily compacted by the passage of solution over the support in a reactor. This compaction reduces the life of the biocatalyst.
Biocatalysts produced by presently known methods for immobilizing enzymes or microorganisms for production of fructose-glucose solutions are limited to operation at temperatures under 50.degree. C. This limits the feed stocks that can be used to provide the sucrose solution. For example, molasses is too viscous at that temperature to be used as a feed stock. Finally, immobilized microorganisms or enzymes prepared by presently known methods remain enzymatically active for a relatively short time, generally less than a month, or must be stored in a sugar solution as in Lazcano, et al.